Embodiments of the present invention relate to power supplies and voltage regulation. More particularly, embodiments of the present invention provide a low current open loop voltage regulator monitor.
Voltage regulators as a part of direct current power supplies are a ubiquitous, if often unseen part of modern life. Almost all electronic devices contain a regulated power supply. Semiconductor devices generally operate at a relatively low direct current voltage, for example 5 volts. Much of the electrical energy to power electronic devices is made available at different voltages. For example, mains power in the United States is nominally 120 volts AC. Automotive power is nominally 12 or 24 volts DC, but is subject to high voltage transients, for example 60 volts, during engine start and other conditions of changing loads.
Power supplies are generally employed to match the requirements of electronic devices (and other types of machines) to the available conditions of electrical power. Many devices, for example hand held electronics, powered by batteries nominally within the voltage range of the electronics employ power supplies to compensate for non-linear discharge characteristics of batteries and to extract as much energy from the batteries as possible.
An important part of most power supplies is a voltage regulator. Voltage regulators function to maintain voltage (and/or current) within a range of output values, for example five volts plus or minus three percent (5 v+/xe2x88x923%). It is generally important to maintain an output voltage within the specified range. Too high a voltage may damage semiconductor devices, leading to decreased reliability or outright failure. If the voltage goes too low, voltage compliance is lost on many components which may lead to several types of failure. In addition, changes in power supply voltage may induce noise into subsequent processing stages of a circuit, diminishing performance or causing errors. A common, undesirable circumstance of suddenly adding a large load to a power supply, for example xe2x80x9cturning onxe2x80x9d a car radio, may xe2x80x9cpullxe2x80x9d the output voltage of a power supply xe2x80x9cdown,xe2x80x9d or out of tolerance. Frequently, power supplies are specifically designed to accommodate a range of such events.
In many voltage regulators, there is a discharge circuit that examines the output voltage to determine if a sudden current pulse will pull the output voltage out of regulation, or beyond the output voltage tolerance limit. Responsive to detecting such an event, additional circuitry may add boost or xe2x80x9csuper chargexe2x80x9d energy to maintain output voltage within acceptable limits. For a variety of reasons, main regulator circuits typically are not able to respond quickly to large transient loads. Additional discharge detection and boost circuitry is generally necessary to overcome limitations in the feedback response of the main regulator circuits.
Discharge circuits may be comprised of a transistor whose emitter is coupled to the output voltage and a base that is coupled to a voltage that is the same as the regulator voltage, but independent of the regulator voltage. It is desirable for these two voltages to always be within a fraction of a VBE (base-emitter voltage) of one another. In this arrangement, the collector of the transistor is coupled to a fast charging circuit that acts independently of the regulator circuit to increase the output voltage.
FIG. 1 shows a prior art discharge circuit as described above that functions as a part of a 5 volt regulator. A zener diode voltage drop across transistor 11 is coupled to the base of transistor 12. The emitter of transistor 12 is coupled to a total of 9 M ohms (resistor 1 and 2) of resistance, further coupled to the diode stack formed by transistor 13 and transistor 14 to ground. The collector of transistor 12 is coupled to the PNP current mirror formed by transistor 15 which gives transistor 11 bias current. The center tap of the resistor pair is coupled to the base of transistor 16. Generally, the collector of transistor 16 is coupled to a fast charging circuit that acts independently of a regulator circuit to increase the output voltage. A voltage difference of a few hundred mV between the base and emitter of transistor 16 will conduct current from a fast charging circuit to support output voltage Vout 20.
If resistors 1 and 2 are of equal value, e.g., approximately 4.5 M ohms each, the voltage on the tap point is approximately 3.9 volts at 50 degrees C., and the tap voltage has a temperature coefficient (T.C.xe2x80x94a measure of how the voltage varies with respect to changes in temperature) of +128 ppm/degree C. when fabricated in a standard bi-polar semiconductor process. If the tap resistors are offset in value, for example resistor 2 is approximately 6 M ohms and resistor 1 is approximately 3 M ohms, the tap voltage is about 4.8 volts at 50 degrees C., and has a T.C. of about +414 ppm/degree C. for the same semiconductor process.
Unfortunately, this prior art design is limited to regulating voltages from about 2.8 volts to about 6 volts. At these voltage extremes, however, the temperature coefficient increases to unacceptable levels. In general, lower temperature coefficients are more desirable.
A further undesirable characteristic of this prior art design is the requirement for about 9 M ohms of resistance. In semiconductor design, this is a very large amount of resistance, requiring a great amount of die area to implement and driving up the cost of products containing this design. Further, newer semiconductor processes are less well suited to making resistances, especially resistances of this scale. For example, a newer semiconductor process may require approximately two to three times as much area to produce the same resistance as prior processes. Such a difference in process renders the prior art design commercially infeasible to produce.
Therefore, a low current open loop voltage regulator monitor with low temperature coefficients and utilizing smaller resistance values is highly desirable.
A low current open loop voltage regulator monitor is disclosed. A circuit is formed with a PTAT current source across a resistor. This current is mirrored by a circuit with two outputs. A first output is formed by a high output impedance current source that has a cascode output. A second output is formed by a higher voltage output current source that has a lower output impedance. The second output feeds an emitter of a PNP device that has its base coupled to the output of the first current source. The output of the first current source and the base of the PNP device are biased above ground by a series of diode and resistor drops that are of the same type comprising the PTAT circuit. This series of devices forms a stack of bandgap voltages that is nominally equal to, but independent of, the regulator output voltage. The emitter of the PNP device is coupled to the base of a transistor whose emitter is coupled to the regulated output, forming a xe2x80x9csuper chargexe2x80x9d circuit that biases the regulator in response to a sudden increase in load current.